This invention relates to an image forming apparatus in which an electrostatic latent image is formed on a photoreceptor by dot exposure wherein the photoreceptor is exposed with a dot-shaped scanning exposure light beam, and more particularly, to an image forming apparatus in which reversal development is conducted to the electrostatic latent image.
In conventional electrophotography, first of all a photoreceptor is charged, by an ordinary method, in order to give photosensitivity to the photoreceptor. Then, the photoreceptor is exposed in order to form a static electric latent image on its surface. Hereinafter, a static electric latent image is called `a latent image`. The latent image is developed by a dry or a wet developer and then transferred to a transfer paper by the action of static electricity or through an intermediary body, and then fixed to form a visible fixed image.
In order to form a visible image by electrophotography, it is necessary to form a latent image on the surface of a photoreceptor which has an excellent electrostatic charging property, dark decay characteristics, light decay characteristics, and gradation property.
A photoreceptor used in a transfer type copier is required to have durability such that gradation and quality of images are not deteriorated because of fatigue even when the photoreceptor is repeatedly used to obtain many copies.
Photoconductive semiconductors used in conventional electrophotography are inorganic photoconductive materials such as selenium, zinc oxide, cadmium sulfide, and cadmium selenide, photoconductive pigments such as phthalocyanine, copper phthalocyanine, cobalt phthalocyanine, and nickel phthalocyanine, and organic photoconductive materials such as poly-N-vinyl carbazole, anthracene, and triallyl amine derivatives. These chemical compounds can be dispersed in the binder resin and/or can form a photoreceptor composed of two layers such as a charge generation layer and a charge transport layer.
Recently, transfer type copiers, especially high speed copiers which can make many copies in a short time, are required in the market. For that reason, a photoreceptor whose light decay is fast, is needed. However, the reciprocity effect appears between the optical attenuation speed of the photoreceptor and the characteristics of charging, dark decay, gradation, image quality, and fatigue and deterioration. As a result, a practically desirable photoreceptor has not been obtained yet.
On the other hand, in response to the requirements of expressing better gradation, especially half tone gradation, a beautiful color image, and excellent image quality, the digital image forming method has come to the fore instead of the conventional analog image method. In the digital image forming method in which dot exposure is conducted by a laser beam, conversion and adjustment of an image can be freely carried out and the image position is accurately aligned, furthermore image data can be transmitted and received. The image quality has been improved to a level equal to that of printed matter.
The LED array method, the EL array method, and the liquid crystal array method can be used to conduct dot exposure. The laser beam method is mainly used at the present time because high speed and high density recording is possible by the laser beam method.
In the case of dot exposure, as well as analog exposure, either the positive exposure system, in which non-image portions are exposed to erase the electric charge on the surface of a photoreceptor in order to form a latent image, or the negative exposure system in which image portions are exposed to erase the electric charge, can be used. However, the negative exposure system is used almost exclusively because of its stability of scanning for writing, excellent reproducibility of dots, and shortened exposure time.
In the case of the negative exposure system, the distribution of light intensity used for each dot exposure is approximate to the normal distribution curve, the peak of which is the center of the dot, and the curve has a long gentle slope with wave patterns according to the intensity of light caused by diffraction. This problem becomes important in the case of a laser beam when the light converges to a diameter of not more than 100 .mu.m, furthermore not more than 50 .mu.m, by the optical lens system. Furthermore, when pulse-duration modulation is used, this problem becomes more serious since the dot diameter substantially becomes smaller. In this case, the exposure distribution of a dot is approximated to Gaussian distribution and the dot diameter indicates the width at which the peak of exposure is 1/e. The diameter of 100 .mu.m corresponds to 16 dot/mm of writing density and the diameter of 50 .mu.m corresponds to 32 dot/mm of writing density.
In the case of the negative exposure system, this wave pattern is remarkable and can be the cause of fog as it acts as a noise. The negative exposure system needs to be combined with reversal development and toner with the same polarity as the electrostatic charge polarity of a photoreceptor must be prepared.
Furthermore, the most appropriate photoconductive semiconductor needs to be selected regarding the light decay curve. Generally speaking, the differential coefficient (the absolute value) of a light decay curve of a photoconductive semiconductor is large in the region of small amounts of light and the electrostatic charge decays sharply. The differential coefficient suddenly becomes small in the region in which the quantity of light is more than a certain amount. As a result, the curve has a gentle slope in this region.
Accordingly, when weak light at the gentle slope portion of the light intensity distribution curve of the above-described spot light, is caught by the electrostatic charge erasing action of the photoreceptor in the region of the small amount of light, the electric potential distribution curve of the formed latent image of a dot becomes funnel-shaped, with a broadly extended edge. Therefore, the peripheral portion of the image developed by toner becomes blurred and fog may also occur.
Those are the reasons why a photoreceptor must be designed to meet the requirements of dot exposure.
Since the dot exposure system has many advantages as described above, it is well-suited to color image forming. charging, image exposure, and developing are repeated many times to superimpose many color toner images on a photoreceptor. The dot exposure system can be applied to this color image forming system. In this color image forming system, when a formed latent image is developed by the contact developing method, the previously formed toner images are damaged. Therefore, there is no alternative but to adopt the non-contact developing method in which a one component or two component developer is used.
In the contact developing method, since the developer comes into contact with the photoreceptor, the toner is attracted to the peripheral portion of the image with the help of the edge effect of the electric field at a latent image on the photoreceptor surface and the density is increased. As a result, sharpness of the image is improved. In the case of non-contact developing, the developer is apart from the photoreceptor. Accordingly, the electric field has a tendency to influence the toner to be drawn to the center of the dot rather than to the peripheral portion of the dot. For that reason, it can not be expected to make the peripheral portion sharp. In order to sharpen the peripheral portion of the dot, it is important that the dot exposure and the optical decay curve have a proper relation.
As described above, a multicolor image can be obtained by repeating exposure, development, and other processes. Accordingly, compared with obtaining a monochrome image, it takes a long time to get a multicolor image and fatigue is increased in the photoreceptor. In order to cope with the situation, it is necessary to reduce the image forming time and to reduce fatigue of the photoreceptor. Conventional technology lacks countermeasures against these points.
In multicolor development, electric potential of the photoreceptor surface is not decreased to the developing level in some cases because of light absorption by the previously adhered toner. This can be the cause of color contamination and deterioration of sharpness. The above-described fact presents a problem of image exposure by visible light.
In the present technical field with regard to toner, the following are important: fluidity of toner to increase static charge property, regularity of toner consumption to eliminate irregular toner consumption in which larger toner particles are consumed first of all and then smaller toner particles are successively consumed, excellent transfer ratio to obtain a high density image, and cleaning property of toner to guarantee repeated copying with excellent reproducibility.
Toner particles are made spherical in order to increase the fluidity of toner, to enhance triboelectric efficiency, and to unify and stabilize static charge property.
However, spherical toner is not always advantageous. Adhesion of toner to the photoreceptor or the developing sleeve is increased by spherical toner particles, and it causes a decrease of the transfer ratio and inferior cleaning of the photoreceptor and the developing sleeve. According to the investigation of the inventors, the more toner particles are spherical, the more toner particles adhere to the above described parts.
The adhesion is caused by physical and electric forces as well as the hardness and evenness of the photoreceptor and the developing sleeve. As far as the photoreceptor is concerned, a part of the reason to cause toner adhesion is the denaturing and softening of the photoreceptor surface caused by corona discharge and oxidation. When contact developing is conducted, the reasons to cause toner adhesion are the toner filming caused by rubbing and Coulomb force acted between the electrostatic charge polarity of the photoreceptor and that of the toner.
The amount of toner which adheres to the photoreceptor and the developing sleeve becomes large, and the toner particles on the photoreceptor and the developing sleeve increase its adhesion, when a period of time between the start and the last operation to clean the photoreceptor and the developing sleeve is long, and the toner adhesion is more repeated.
When the amount of adhesion and the adhesion strength are increased, the transfer ratio decreases, and it can be the cause of inferior cleaning.